1. Field of the Invention
The present invention relates to a small-sized antenna unit mounted on a moving body, for example, vehicles or the like and suitable as antenna units for ITS (Intelligent Transport System) such as ETC (Estimated Time of Completion), VICS (Vehicle Information Communications System), or the like, or antenna units for GPS (Global Positioning System).
2. Description of the Related Art
The prior art of such kind of antenna unit is described in Japanese Unexamined Published Patent Application No. H9-223912 and so on. Such prior art will be described with reference to a cross sectional view of FIG. 5. An antenna unit shown in the figure is mainly composed of a circuit board 1 provided on a bottom surface thereof with an amplifier 2, an antenna element 3 mounted on the circuit board 1, and a shield casing 4 of metallic sheet mounted on the circuit board 1 to shield the amplifier 2.
A ground conductor 5 is provided on a substantially whole roof surface of the circuit board 1 to be used as a ground for the antenna element 3. The antenna element 3 is called a patch antenna to comprise a patch electrode 7 having a predetermined shape (for example, substantially square) and provided on a roof surface of a dielectric board 6, which is square as viewed in plan view. An upper end of a feeding pin 8 extending through the dielectric board 6 is soldered to a feeding point of the patch electrode 7. Since a lower end of the feeding pin 8 is soldered to the amplifier 2, the patch electrode 7 and the amplifier 2 are electrically connected to each other. A ground electrode 9 is provided on a substantially whole bottom surface of the dielectric board 6, and the antenna element 3 is placed on the circuit board 1 in a state, in which the ground electrode 9 is brought into close contact with the ground conductor 5. The shield casing 4 is formed to have a box-shape with a top opening, and mount pieces 4a provided projectingly in a plurality of locations on a peripheral edge of the opening are inserted through the circuit board 1 to be bent whereby the shield casing 4 is mounted on the circuit board 1 in a state, in which it covers the amplifier 2.
With the antenna unit schematically constructed in this manner, a feeding cable such as coaxial cable or the like is connected to the amplifier 2, so that when a predetermined high frequency signal is fed to the patch electrode 7 via the amplifier 2, circular polarization or linear polarization is emitted from the patch electrode 7. Also, a signal received by the patch electrode 7 is output to a receiver circuit via the amplifier 2 and the feeding cable. In addition, in the case where the shield casing 4 covers the amplifier 2 as in the antenna unit, high reliability can be ensured since there is sharply reduced the possibility that antenna characteristics is deteriorated by undesirable radio waves radiated from the amplifier 2 and interfering waves from outside adversely affect the amplifier 2.
With the antenna unit of the prior art shown in FIG. 5, while the antenna element 3 is fixed to the circuit board 1 by soldering both ends of the feeding pin 8 to the patch electrode 7 and the amplifier 2, torque with the feeding pin 8 as an axis of rotation acts on the antenna element 3, which is fixed only at the feeding point, so that vibrations during traveling make the antenna element 3 liable to fluctuate on the circuit board 1 in a direction of rotation, such fluctuation giving rise to a cause for making antenna characteristics unstable. Also, when the antenna element 3 fluctuates in the direction of rotation, stress such as torsion or the like acts on the feeding pin 8 and its soldered portions to cause a problem that a danger of failure in conduction is increased in likelihood and lengthening of life is hard to achieve.